A common type of air brake system for trucks utilizes an actuator associated with each brake. The actuator has an opening in one side through which a push rod extends, the push rod being in turn connected to the actual vehicle brake through a suitable mechanical linkage.
When the vehicle brakes are not being applied, a spring or the like within the actuator holds the push rod in a relatively retracted position in which the brake is off. However, when an operator applies the brakes, pressurized air is supplied to the actuator, the pressurized air causing the push rod to move along its longitudinal axis, in a direction out of the actuator. This movement continues until the brake engages and provides a return force that balances the force provided by the air pressure.
As the brake wears, the push rod must move through a larger and larger distance from its retracted position to an extended position in which the brake is applied. If the process continues uncorrected, the push rod travel will gradually reach its upper limit, determined by the geometry of the actuator. When this occurs, the brakes will not be effective, even when fully applied by the vehicle operator. Because of this, in the U.S., trucking regulations place an upper limit of two inches on the maximum amount of push rod travel between its retracted and extended positions.
Two basic types of solutions have been provided to address the problem of wear in air brake systems. The first of these solutions is a device known as a slack adjuster that is included in the linkage between the push rod and the brake. A slack adjuster removes slack from this linkage, thereby reducing the amount of push rod travel necessary before the brakes are applied. Both automatic and manual slack adjusters are well known and in use. In an automatic slack adjuster, the slack removal operation is automatic, and occurs whenever a predetermined degree of slack develops in the linkage. However, an automatic slack adjuster is a relatively expensive and complex device, and is subject to occasional malfunction. A manual slack adjuster must be adjusted manually whenever sufficient push rod travel develops in the brake system. However in general, an operator has no means of readily determining when such wear exists.
In order to overcome the limitations of existing slack adjusters, a variety of brake wear indicators have been devised to provide an indication to an operator that excess push rod travel has developed. For example, U.S. Pat. No. 4,279,214 describes a break wear indicator that comprises a sleeve that extends around the push rod and through the opening in the actuator through which the push rod extends. A relatively loose fit is provided between the indicator and the push rod, and a comparatively snug fit is provided between the indicator and the actuator. When the brakes are applied, the indicator is forced out of the actuator along with the push rod. When the brakes are subsequently released, the fit between the indicator and the actuator prevents the indicator from retracting. Thus at any given time, visual inspection of the indicator will indicate the maximum travel of the push rod since the indicator was last reset into its retracted position. While effective in principle, the brake wear indicator proposed in this patent suffers from the problem that there is considerable variation in the size of the opening in the side of the actuator through which the push rod extends. Thus, in practice, indicators of various outside diameters must be provided in order to cover the different brands and models of actuators currently in use.
Another prior brake wear indicator is described in U.S. Pat. No. 4,757,300. In one embodiment described therein, a block containing a magnetically sensitive read switch is mounted to the actuator, and a magnet is mounted to the push rod. These elements are so positioned such that excessive push rod travel causes the magnet to change the reed switch position, actuating visual and audio alarms in the cabin of the vehicle. This is thus a relatively complex and expensive solution to the problem, and is potentially difficult to maintain in correct adjustment and calibration.
A third prior brake wear indicator is described in U.S. Pat. No. 4,776,438. In this device, the actuator consists of an arm that is secured to the vehicle frame or actuator, the arm including a pointing indicator that can be set at a desired position along the length of the arm. The pointing indicator provides a visual indication of a farthest safe travel of the push rod when the brakes are applied. Although simple in construction, this brake wear indicator suffers from the problem that different readings can be obtained depending upon the angle at which the indicator and push rod are viewed.